Study Of Crack Propagation Based On Infrared Thermal Imaging Technology

Fatigue fracture is one of the most main failure modes in engineering practice.It often leads to disastrous consequences since there is no obvious sign before the sudden fracture.Considering the fatigue fracture of an important component can cause huge economic loss,so researchers pay attention to the fatigue fracture.Crack length and fatigue crack growth rate(FCGR)are two important parameters of the fatigue fracture.It is of great significance to accurately estimate the crack length and crack propagation rate.The infrared thermal imager is a relatively new temperature measuring instrument with many advantages of non-contact,high sensitivity and accuracy,which is widely applied to the fatigue research including obtaining fatigue limit and the S-N curve in a short time.However,at present,the study of crack propagation based on infrared thermal imaging technology is in the beginning stage.From the point of view of energy,fatigue crack propagation is an energy dissipation process,which leads to the temperature rise during propagation process.In general,fatigue crack growth rate will increase with the increase of the energy dissipation rate.The position of the largest intrinsic dissipation source and the trend of dissipative energy evolution at the crack tip can be accurately measured using the infrared thermal imager.In this way,the path of crack propagation,crack length and crack growth rate are easily estimated.In addition,the above-mentioned characteristics of infrared thermal imager contribute to analyze the mechanism of crack propagation.However,the research on crack growth based on infrared thermal imaging technology has not been put enough attention.In this paper,the research on the crack growth and fatigue damage evaluation is divided into three aspects:(1)Crack propagation is systematic studied based on fracture mechanics.Make a systematic summarize of characters and influencing factors of crack propagation.Fatigue crack growth tests are conducted for 316 L steel under the varied stress ratios.And the experimental results of the crack growth test are analyzed.(2)Crack propagation is systematic studied based on thermodynamics.Some thermodynamics and methods of measuring crack length are introduced systematically.The location of the largest internal heat source is near crack tip since the plasticity deformation amount of crack tip is largest.The crack length and path of crack growth are estimated by recording the locations of the largest internal heat source at different time.Based on this principle,fatigue crack growth test is carried out to estimate the length of crack and the path of crack growth through the infrared t hermal imager.Crack length measured by infrared thermography are verified by crack length measured by COD gauge.(3)An energy dissipation-based fatigue crack growth rate model is firstly established,which can be described that the crack propagation rate is linearly related to the energy dissipation rate.Fatigue crack growth tests are conducted on the 316 L steel under the varied stress ratios to verify the proposed model.The experimental results verify that crack propagation rate is linearly related to energy dissipation rate except for the proportional coefficient of the model varied with different stress ratios of fatigue crack growth tests.An effective energy dissipation rate is put forward to replace the energy dissipation rate in the model which correct the model.